Fractionation control



J y 0, 1954 D. M. BOYD, JR 2,684,326

FRACTIONATION CONTROL Original Filed May 11, 1949 INVENTOR DAVID M. BOYD JR.

AT TOR N EYSI Patentecl July 20, 1954 UNITED STATES FATENT OFFICE FRACTIONATION CONTROL David M. Boyd, Jr., Clarendon Hills, 111., assignor to Universal Oil Products Company, Chicago, Ill., a corporation of Delaware 7 Claims.

1 This application is a continuation-impart of my previously filed application, Serial No. 92,559,

filed May 11, 1949, now Patent No. 2,580,651,

January 1, 1952.

This invention relates to an improved method and means for operating a fraotionating column to obtain the maximum separation of components for a given amount or" heat consumption. More specifically, the present invention concerns auto.- matic means for maintaining a substantially uniform temperature and composition gradient throughout the height of a fractionating column by the novel combined use of indicating and control means connecting with the column both above and below the feed point thereto and as a result provides improved efficiency from each of the trays of the column.

In a usual fractionating operation, the mixed or multi-component fluid stream is charged to approximately the mid-point of the column, which is a vertically disposed chamber having a plurality of spaced bubble trays or decks, or alternatively, a substantial depth of suitable packing material which will effect a desirable mixing of the counter-currently flowing liquid and vapor streams within the column. Heat is normally supplied to the column by means of a reboiler which connects with the lower portion of the column and to a liquid reservoir maintained therein by means of suitable level control means, while reilux is normally returned to the top of the column in a regulated quantity suitable to permit proper rectification within the upper portion of the column. Also, in present types of continuously operating fractionating column and control systems, a temperature or composition indicating and controlling device is connected to the interior of the column at either the stripping section or the rectification section, and the heat input to the column is regulated by this temperature controller through either varying the heat input at the reboiler, or changing the quantity of reflux returned to the top of the column. Alternatively, a material balance con-- trol system. is sometimes utilized which provides a flow controller on the bottoms product that varies with changes in the flow rate in the feed stream to the column. Still another control method, is the use of a reference material at a selected control point in the column, whereby the differential between the vapor pressure in the column and that of the reference fluid controls the heat supplied to the fractionating column. Inasmuch as temperature is one means of measuring compositions, either temperature or composition measuring means may be utilized to provide an indication of temperature and composition gradient within the column.

While the foregoing methods of controlling fractionating by automatic means are in general satisfactory to effect fractionation and separation of the components of the feed stream, and to provide an overhead product of substantially constant composition, such methods do not necessarily maintain a desirable uniform tempera ture and composition gradient throughout the length of the column. In the event that the column is operating such as to move too much material overhead, the temperature gradient curve of the column will have a steep vertical rise just above the feed tray, while conversely, the column is operating so as to remove an insufficient amount overhead, the curve will have a steep vertical dip in it just below the feed tray. These vertical slopes in the temperature gradient curve column indicate that the trays in that zone are not operating at their maximum efficiency, and the given tower or column is not providing the maximum separation of the components for a given amount of heat consumption.

It is a principal object of the present improved invention to utilize differential temperature or composition measuring means both above and below the feed point of the column in a mamier to measure differences in temperature gradient which may occur during operation, and to effect a control of the heat distribution to both the upper and lower portions of the column, whereby to provide a uniform temperature gradient and a maximum operating efficiency therein.

It is also an object of the present invention to control heat input to both the upper and lower portions of a fractionating column by means of control instruments which in turn connect with and are actuated by spaced temperature indicating means above and below the feed point to the column.

Broadly, the present improved arrangement provides a method and means for operating a fractionating column to effect the separation of a mixed feed stream, wherein at least overhead and bottom components are withdrawn from the column, in a manner which comprises, maintaining a pair of temperature indicating means at vertically spaced points above the level of introducing the feed stream to the column, maintaining a pair of temperature indicating means at vertically spaced points below the feed level to the column, measuring temperature gradient above and below said feed level from said indidating means, measuring the differential between the temperature gradient above the feed level and the temperature gradient below the feed level and regulating the heat input to both the upper and lower portions of said column responsive to both a change in the differential between the temperature gradients and to a change in the gradient of one of said pairs of temperature indicating means.

In a preferred arrangement of the invention, automatic temperature or composition indicating and control means are used within the column as well as exteriorally thereof, such that an automatic indication and comparison of the temperature gradients is made in a continuous manner and so that the heat input to the column may be regulated continuously in a manner providing the desired uniform temperature and gradient throughout the entire length of the column, with a resulting improved efficiency in the separation of the desired components. The comparison controller may be utilized to vary and control the heat distribution to either the upper or lower portions of the column. For example, the controller may connect with the reflux line to the column and thus vary the quantity of cool reflux medium being returned to the upper portion of the column, in which case, an additional control instrument connects with the temperature gradient measuring instrument, which in turn connects with the temperature indicating means within the lower part of the column, and this latter control instrument connects with the reboiler or other heat supplying means connecting with the lower portion of the column. Alternatively, the comparison controller may connect with the heat supply means, or reboiler, at the lower portion of the fractionating column in order to effect a direct change in the heat supply to the lower portion of the column, while the additional control means is provided to connect with a flow control valve in the reflux line and regulate the quantity of cool reflux being returned to the upper portion of the column. This additional controller being connected to and controlled by the temperature differential measuring instrument, which in turn connects directly with the spaced temperature indicating means within the upper portion of the fractionating column.

While the operation of a preferred embodiment of the invention is efiected through the use of automatic control instruments, it is not intended to limit the operation to the use of any one particular type of instrument or controller. lhe temperature or composition indicating means and the temperature differential measuring means may comprise any of the conventional types which are utilized in processing equipment and the output signals from such instruments may be indicated and compared by either elecrical or pneumatic means, as utilized by the common types of instruments familiar to those persons who are designers and operators of processing units. In addition to temperature indicating devices, various types of vapor pressure devices or other means of indicating composition, may be utilized.

Reference to the accompanying drawing and the following description thereof will serve to further clarify the operation and the use of apparatus in the present improved fractionation control system, as well as point out additional features and advantages which may be derived therefrom.

Figure 1 of the drawing shows digrammatically 4 a fractionating column which utilizes automatic means for closely controlling the temperature and composition gradient throughout, in accord ance with one embodiment of the present invention.

Figure 2 of the drawing shows diagrammatically a fractionating column utilizing an alternative embodiment of the present invention for closely controlling and maintaining a uniform temperature and composition gradient throughout the height of the column.

Referring now to Figure l of the drawing, there is shown a feed line I, having a suitable flow control valve 2, such that a mixed or multiple component feed stream may be introduced into the intermediate portion of the fractionating column 3. The fractionating column 3 is indicated as a vertically disposed column having a plurality of bubble decks or trays i at vertically spaced intervals and in a quantity suflicient to provide fractionation and separation of the feed stream. In the present embodiment, it may be assumed that the pressure is maintained substantially constant within the column and that the feed stream is regulated at a substantially constant flow rate by means of an automatic flow control valve 2. A body of liquid is maintained in the lower portion of the column 3 by means of a level controller 5 which in turn regulates the rate of liquid withdrawal from the bottoms outlet line 3, having control valve The net heat input to the column is provided through a reboiler 8, which in turn is connected to the lower portion of the column by means of lines 9 and it]. In this instance, heat is supplied to the reboiler 8 by means of steam or other hot fluid medium passing through line ll having control valve l2, and being discharged through outlet line [3.

The overhead product stream from the fractionating column 3 is passed by way of line id to a condenser i5 and to a condensate receiver l6. Uncondensed vapors and gases are vented through line li and control valve it, while the resulting condensed overhead product stream is withdrawn from the lower end of receiver it by way of line 19 and pump 25. A portion of the liquid overhead is returned to the upper end of the column 3 as reflux, through line El and flow control valve 22, while the remaining portion of the condensate passes through line 23 and flow control valve 2% to storage, or to other process equipment. The quantity of condensate passing by way of line 23 and valve 2t, in this par-- ticular embodiment, depends upon th operation of a liquid level controller 25 connecting with receiver [6.

In accordance with the present invention, two temperature measuring devices a and b are placed at vertically spaced points above the feed inlet to the column such that variations in the temperature gradient above the feed point may be noted and compared. Also, two temperature indicating devices 0 and d are placed at vertically spaced points below the feed point of the column such that variations in the temperature gradient below the feed point may be noted and compared. The temperature indicating devices a and 1) connect through suitable lines or transmitting means 26 and 21 to a differential temperature measuring instrument 28. Likewise, the temperature measuring or indicating means 0 and (1 connect through suitable lines or transmitting means 29 and St! to a differential temperature measuring instrument 3|. The differential temperature measuring instruments 28 and 3! in turn transmit changes in temperature gradient above and below the feed level to the column through suitable lines 32 and 33 to a control instrument 34, which in turn is suitable to receive impulses corresponding to changes in temperature gradient and compare them for regulating heat input to the column. In other words, the control instrument 3 loperates responsive to comparisons and variations in temperature differential transmitted thereto from the temperature gradient instruments 28 and SI.

In the present embodiment, the control instrument 3 connects by means of a line or transmitting means 35 directly to the flow control valve 22 in the reflux line 2|, and thus operates to regulate the quantity of reflux passing to the top of column 3 and vary the heat thereto as is dictated by the changes in temperature gradients above below the feed point to the column. Alternatively, the control instrument 34 may connect with a pneumatic set flow controller, which in turn would regulate the flow control valve 22. As a means of further controlling the heat input to the column, an additional control instrument 36 connects by means or" line 3'? with the transmitting line 33 so as to operate responsive to a change in temperature gradient within the lower portion of the fractions/ting column, and in this instance being indicated by the transmitting indicators 0 d and instrument 3i. The control instrument 35 in turn connects through line 38 with the flow control valve E2, in the heat supply line i i, so as to regulate the heat input into the lower portion of column 3, through reboiler 8, as dictated by variations in temperature gradient in the lower portion of the column. Here again, the control instrument 36 may connect with a pneumatic set flow controller, which in turn would regulate the flow control valve l2, This additional control instrument insures a closer temperature control on the fractionating column in that it provides temperature control within the lower portion of the column in addition to the overall control of heat distribution of the column as provided by controller 34 and reflux valve 22. For example, the control instrument 36 in the present arrangement would insure that substantially none of the lower boiling product would be carried into the bottom portion of the column and be Withdrawn through the bottoms line 6.

It may be further explained and noted that where the diflferential temperature between points a and b is small or substantially less than between points 0 and d, that the comparison instrument 34, connecting to the valve 22, operates in a manner to open valve 22 and increase the quantity of reflux, to in turn reduce the temperature within the stripping section of the column and provide a reduction in the amount of overhead product being withdrawn, Conversely, where the temperature differential between points c and d is small or substantially less than between points a and '0, then the comparison and regulating device 3t operates to decrease the quantity of reflux permitting an increase in temperature within the stripping section of the column and cause the production of more overhead from the column. Simultaneously, with the operation of controller 34 to reduce the reflux to the column, the controller 36 operates valve [2 responsive to the temperature difierential measuring instrument 3!, to increase the heat input to the lower portion of the column and bring back a more uniform temperature gradient in the lower portion thereof, and insure that none of the overhead product stream is permitted to reach the bottom of the column.

Referring now to Figure 2 of the drawing, there is indicated a fractionating column 3, which is similar to that shown in Figure 1 of the drawing, having an intermediate feed point for a mixed or multiple component feed stream entering by way of line I and valve 2. Overhead product is indicated as being withdrawn by way of line i l and reflux introduced by way of a line 21' having a suitable control valve 22, while bottoms product is withdrawn by way of line 6' having automatically controlled valve 1'. Also in a manner similar to Figure l of the drawing, heat is supplied to the lower portion of the column 3' by way of a reboiler 8, which in turn is supplied with heat through line H and an automatically controlled valve I2.

In this instance, two temperature measuring devices a and h are placed at vertically spaced points above the feed inlet to column 3, and two vertically spaced temperature measuring devices c and d are placed below the feed point to the column, such that variations in the temperature gradient both above and below the feed level may be noted and compared to provide a portion of the regulation of the heat distribution to the column. A temperature measuring instrument 28 connects with the temperature sensitive means a and b, while another temperature gradient measuring instrument 3 l connects with the temperature sensitive devices c and d. Also in a manner similar to the embodiment of Figure l, a temperature gradient comparison and control instrument 34' connects with each of the diiierential temperature measuring instruments 28 and ti. However, in this alternative embodiment, the controller 34' connects through a line or transmitting means 39 to an instrument 4 I, such as a pneumatic set flow controller, which in turn connects to the control valve I? in heat supply line ll. lhus, the heat input to the column, through reboiler 8', is regulated in direct response to the variations in the temperature gradients in the column, both above and below the feed point thereto, as measured and indicated through the temperature sensitive means a, b, c and d.

An additional controller 36. connects through a line 3'! with line 32' and the temperature differential measuring instrument 28, so that it may operate responsively to variations in temperature gradient between points a and b within the upper portion of the fractionation column. The control instrument 3&3 connects through line ii] with an instrument :52, such as a pneumatic set flow controller, which in turn connects to the control valve 22 in reflux line it, and may thus vary the quantity of reflux being returned to the top of the column.

In effecting the controlled operation of the fractionating column in accordance with the present alternative embodiment, where the differential temperature between points a and b is small or substantially less than between points 0' and d, then the comparison instrument 34' operates valve [2 in a manner to reduce the heat input to the reboiler 8' and in turn reduce the temperature within the stripping section of the column to provide a reduction in the amount of overhead product being withdrawn. At the same time controller 36' operates valve 22', responsive to the temperature differential being indicated by instrument 28', and the quantity of reflux to the column through line 2 l is increased to lower the temperature within the upper portion of the column.

Conversely, where the temperature difierential between points and d is small or substantially less than between points a and b, then the comparison instrument 34 operate valve [2 to increase the heat input to the reboiler 8' and to the lower portion of the column, while at the same time controller 36 acting responsive to the temperature differential between points a and b and to the temperature difierential instrument 28, operates valve 22 to decrease the quantity of reflux being returned to the top of the column, and thus permit an increase in temperature within that portion of the column.

It is generally agreed that single point temperature measuring device or single control point instruments and their accompanying control systems, as used in many fractionating columns, do not avoid in any direct manner the steep vertical slopes in the temperature gradient curves and as a result do not control composition above and below the feed tray, as is accomplished by the present method of operation. By controlling temperature gradient in a close manner both above and below the feed level, as accomplished by the present embodiments, the trays above and below the feed point are maintained at a high operating efficiency and as a result there is a maximum amount of separation of components in the feed stream for a given amount of input to the column.

It may again be pointed out that it is not in tended to limit the present improved operation to the use of any one type of control instrument or instruments, for any of the suitable and customary types of electrical or air-controlled instruments that measure temperature or composition differences, and will provide regulation and control in accordance with differential gradients may be used within the scope of the invention.

Further, it is not intended to limit the spacing or positioning of the temperature or composition sensitive devices at the points above and below the feed level, for obviously, such points are preferably located so that they will provide a measure of temperature and composition gradient in the critical portions of the column above and below the feed level. This spacing will vary in accordance with the size of the column, as well as with the type of mixture which is fractionated therein.

I claim as my invention:

1. In the fractionation of a mixed feed stream within a fractionating column wherein at least overhead and bottom components are withdrawn, the improved method of obtaining a substantially uniform temperature and composition gradient throughout the column which comprises, maintaining a pair of temperature indicating means at vertically spaced points above the level of introducing said feed stream to the column, main-- taining a pair of temperature indicating means at vertically spaced points below said feed level to the column, measuring temperature gradient above and below said feed level from said indicating means, measuring the differential between the temperature gradient above the feed level and the temperature gradient below the feed level, regulating the heat input to one end of said column responsive to changes in said differential between said temperature gradients and regulating the heat input to the other end of the column responsive to the change in the gradient of one of said pairs of temperature indicating means.

2. In the fractionation of a mixed feed stream within a fractionating column wherein at least overhead and bottom components are withdrawn, the improved method of obtaining a substantially uniform temperature and composition gradient throughout the column which comprises, maintaining a pair of temperature indicating means at vertically spaced points above the level of introducing said feed stream to the column, maintaining a pair of temperature indicating means at vertically spaced points below said feed level to the column, measuring temperature gradient above and below said feed level directly from said indicating means, measuring the differential between the temperature gradient above the feed level and the temperature gradient below the feed level and regulating the heat distribution to the upper portion of said column responsive to changes in said differential between said tempera ture gradients while simultaneously regulating the heat input to the lower portion of said column responsive to a change in temperature gradient below said feed level measured by said lower pair of temperature indicating means,

3. The method of claim 2 further characterized in that the differential between the tem perature gradients above and below the feed level regulates the quantity of reflux material returned to the upper portion of said column as the means for regulating the heat distribution thereto.

4. In the fractionation of a mixed feed stream within a fractionating column wherein at least overhead and bottom components are withdrawn, the improved method of obtaining a substantially uniform temperature and composition gradient throughout the column which comprises, maintaining a pair of temperature indicating means at vertically spaced points above the level of introducing said feed stream to the column, maintaining a pair of temperature indicating means at vertically spaced points below said feed level to the column, measuring temperature gradient above and below said feed level directly from said indicating means, measuring the differential between the temperature gradient above the feed level and the temperature gradient below the feed level and regulating the heat input to the lower portion of said column responsive to changes in the difierential between said temperature indicators while simultaneously regulating the quantity or reflux material returned to the upper portion of said column responsive to a change in temperature gradient above said feed level and measured by said upper pair of temperature indicating means.

5. In a fractionating column having a feed inlet at an intermediate point in the height thereof, the combination of a pair of temperature indicators at vertically spaced points above said inlet, a pair of temperature indicators at vertically spaced points below said inlet, a temperature gradient measuring device connected to each of said pairs of temperature indicators, means co-acting with said devices for regulating the heat distribution to one end of said column in response to the differential between the temperature gradient above said inlet and the temperature gradient below said inlet, and additional means co-acting with one of said devices regulating the heat distribution to the other end of said column in response to a variation in tem perature gradient therein and indicated by the pair of temperature indicators in that portion of the column.

6. In a fracitionating column having a feed inlet at an intermediate point in the height thereof, the combination of a pair of temperature indicators at vertically spaced points above said inlet, a pair or" temperature indicators at vertically spaced points below said inlet, a temperature gradient measuring device connected to each of said pairs of temperature indicators, means coacting with said devices for regulating the supply of heat to the upper portion of the column in response to the differential between the temperature gradient above said inlet and the temperature gradient below said inlet, and additional means co-acting with said device connecting to the pair of temperature indicators at spaced points below said inlet controlling the heat input to the lower portion of said column.

7. In a fractionating column having a feed inlet at an intermediate point in the height thereof, the combination of a pair of temperature indicators at vertically spaced points above said inlet, a pair of temperature indicators at vertically spaced points below said inlet, a temperature gradient measuring device connected with each of said pairs of temperature indicators, means co-acting with said devices for regulating the supply of heat to the lower portion of said column in response to the difierential between the temperature gradient above said inlet and the temperature gradient below said inlet, and additional means co-acting with said device connecting to said spaced pair of temperature indicators above said inlet controlling the quantity of reflux returned to the upper portion of said column.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,587,188 Schneible June 1, 1926 1,815,129 Peters July 21, 1931 2,022,899 Kramer Dec. 3, 1935 2,252,550 Bragg Aug. 12, 1941 2,386,831 Wright Oct. 1.6, 19 15 2,580,651 Boyd, Jr Jan. 1, 1952 OTHER REFERENCES Automatic Control of Fractionating Towers, by V. V. St. L. Tivy, Oil and Gas Journal, November 25, 1948. 

1. IN THE FRACTIONATION OF A MIXED FEED STREAM WITHIN A FRACTIONATING COLUMN WHEREIN AT LEAST OVERHEAD AND BOTTOM COMPONENTS ARE WITHDRAWN, THE IMPROVED METHOD OF OBTAINING A SUBSTANTIALLY UNIFORM TEMPERATURE AND COMPOSITION GRADIENT THROUGHOUT THE COLUMN WHICH COMPRISES, MAINTAINING A PAIR OF TEMPERATURE INDICATING MEANS AT VERTICALLY SPACED POINTS ABOVE THE LEVEL OF INTRODUCING SAID FEED STREAM TO THE COLUMN, MAINTAINING A PAIR OF TEMPERATURE INDICATING MEANS AT VERTICALLY SPACED POINTS BELOW SAID FEED LEVEL TO THE COLUMN, MEASURING TEMPERATURE GRADIENT ABOVE AND BELOW SAID FEED LEVEL FROM SAID INDICATING MEANS, MEASURING THE DIFFERENTIAL BETWEEN THE TEMPERATURE GRADIENT ABOVE THE FEED LEVEL AND THE TEMPERATURE GRADIENT BELOW THE FEED LEVEL, REGULATING THE HEAT INPUT TO ONE END OF SAID COLUMN RESPONSIVE TO CHANGES IN SAID DIFFERENTIAL BETWEEN SAID TEMPERATURE GRADIENTS AND REGULATING THE HEAT INPUT TO THE OTHER END OF THE COLUMN RESPONSIVE TO THE CHANGE IN THE GRADIENT OF ONE OF SAID PAIRS OF TEMPERATURE INDICATING MEANS. 